The invention is based on a drive circuit for LEDs and an associated operating method as claimed in the preamble of claim 1. This relates in particular to reducing the drive power losses in light-emitting diodes (LEDs) by means of a pulsed LED drive circuit.
As a rule, series resistors are used for current limiting when driving light-emitting diodes (LEDs), see, for example, U.S. Pat. No. 5,907,569. A typical voltage drop across light-emitting diodes (UF) is a few volts (for example, for Power TOPLED UF=2.1 V). The known resistor Rv in series with the LED (see FIG. 1) produces a particularly high power loss, particularly if the battery voltage UBatt is subject to major voltage fluctuations (as is normal in motor vehicles). The voltage drop across the LEDs still remains constant even when such voltage fluctuations occur, that is to say the residual voltage across the series resistor Rv falls. Rv is thus alternately loaded to a greater or lesser extent. In practice, a number of LEDs are generally connected in series (in a cluster) in order to achieve better drive efficiency (FIG. 2). Depending on the vehicle power supply system (12 V or 42 V), a large number of LEDs can accordingly be combined to form a cluster. With a 12 V vehicle power supply system, there is a lower limit on the battery voltage UBatt down to which legally specified safety devices (for example the hazard warning system) must be functional. This is 9 volts. This means that, in this case, up to four Power TOPLEDs can be combined to form a cluster (4xc3x972.1 V=8.4 V).
The power loss in the series resistor is converted into heat, which leads to additional heatingxe2x80x94in addition to the natural heating from the LEDs in the cluster.
The technical problem is to eliminate the additional heating (drive power loss from the series resistors). There are a number of reasons for this. Firstly, enormous losses occur in the series resistor; in relatively large LED arrays, this can lead to a power loss of several watts. Secondly, this heating from series resistors itself restricts the operating range of the LEDs. If the ambient temperature TA is high, the maximum forward current IF=f (TA) must be reduced in order to protect the LEDs against destruction. This means that the maximum forward current IF must not be kept constant over the entire ambient temperature range from 0 to 100xc2x0 C. In addition, when LEDs with series resistors are being operated, another problem is the fluctuating supply voltage, as is frequently the case in motor vehicles (fluctuation from 8 to 16 V with a 12 V power supply system; fluctuation from 30 to 60 V with the future 42 V vehicle power supply system). Fluctuating supply voltages lead to fluctuating forward currents IF, which then result in different light intensities and, associated with this, fluctuations in the brightness of the LEDs.
In the past, series resistors have always been used to limit the forward current through the LEDs. In most cases, the same board has been used for all the series resistors and, if possible, this has been mounted at a suitable distance from the LEDs. This distance was chosen so that the heating from the series resistors Rv did not influence the temperature of the LEDs.
A further problem is the choice of the maximum forward current IF of LEDs. When operating LEDs with series resistors Rv, the maximum permissible forward current IF cannot be chosen, since the forward current must be reduced if the ambient temperature TA is higher. A forward current IF is therefore chosen which is less than the maximum permissible current (FIG. 3). This admittedly increases the temperature range for operation of the LEDs, but does not utilize the forward current IF optimally. The example in FIG. 3 (Power TOPLED, Type LA E675 from Siemens) shows the forward current IF as a function of the ambient temperature TA. The maximum forward current IF may in this case be 70 mA up to an ambient temperature of 70xc2x0 C. Above an ambient temperature of 70xc2x0 C., the forward current IF must then be reduced linearly, until it is only 25 mA at the maximum permissible ambient temperature of 100xc2x0 C. A variable series resistor Rv would have to be used for optimum utilization of this method of operation of LEDs.
A further problem is voltage fluctuations. Until now, there have been no drive circuits for LEDs in practical use in order to prevent voltage fluctuations, and thus forward-current fluctuations (brightness fluctuations). They therefore have had to be tolerated by necessity.
The object of the present invention is to provide a drive circuit for an LED as claimed in the preamble of claim 1, which produces as little emitted heat and power loss as possible.
This object is achieved by the distinguishing features of claim 1. Particularly advantageous refinements can be found in the dependent claims.
A pulsed LED drive is used in order to eliminate the series resistor Rv and thus the high drive power loss. FIG. 4a shows the principle of pulsed current regulation for LEDs. A semiconductor switch, for example a current-limiting power switch or, preferably, a transistor T (in particular of the pnp type, although the npn type is also suitable if a charging pump is also used for the drive), is connected by its emitter to the supply voltage (UBatt) (in particular the battery voltage in a motor vehicle) . When the transistor T is switched on, a current iLED flows through the LED cluster (which, by way of example, in this case comprises four LEDs), to be precise until the transistor T is switched off again by a comparator. The output of the comparator is connected to the base of the transistor. The one (positive) input of the comparator is connected to a regulation voltage, and the second (negative) input of the comparator is connected to a frequency generator (preferably a triangle waveform generator with a pulse duration Tp and, accordingly, a frequency 1/Tp, since this has particularly good electromagnetic compatibility, although other pulse waveforms such as a sawtooth are also possible). The transistor T is switched on if the instantaneous amplitude of the triangle waveform voltage UD at the comparator is greater than the regulation voltage UReg. The current which flows is iLED. When the instantaneous amplitude of the triangle waveform voltage falls below the constant value of the regulation voltage UReg on the comparator, the transistor T is switched off again. This cycle is repeated regularly at the frequency f at which the triangle waveform generator operates.
The current flowing via the LEDs is pulsed in this way (FIG. 4b). The square-wave pulses have a pulse width which corresponds to a fraction of Tp. The interval between the rising edges of two pulses corresponds to Tp.
The LEDs are connected in series with a means for measuring the current (in particular a measurement resistor RShunt between the LEDs and ground (case 1) or else between the semiconductor switch (transistor T) and the terminal of the supply voltage UBatt (case 2)). The pulsed current iLED is tapped off on the measurement resistor RShunt. The mean value of the current {overscore (i)}LED is then formed via an auxiliary means. The auxiliary means is, for example, an integration means (in case 1), preferably an RC low-pass filter, or a differential amplifier (in case 2). This mean value is used as the actual value for current regulation, and is provided as an input value to a regulator (for example a PI or PID regulator). A nominal value, in the form of a reference voltage (URef) for current regulation is likewise provided as a second input value to the regulator. The regulation voltage UReg at the output of the regulator is set by the regulator such that the actual value always corresponds as well as possible to the nominal value (in terms of voltage). If the supply voltage UBatt varies due to fluctuations, the on-time of the transistor T and the length of the square-wave pulse (FIG. 4b) are also adapted as appropriate. This technique is known per se as PWM (pulse-wave modulation).
The advantage of pulsed current regulation for LED clusters is primarily the rapid compensation for supply fluctuations in UBatt by means of PWM. The mean value of the LED current ({overscore (i)}LED) thus remains constant. There are thus no longer any brightness variations in the LEDs when voltage fluctuations occur. A further advantage is protection against destruction resulting from an increased temperature, as explained above (as a function of the ambient temperature TA).
The circuit according to the invention advantageously allows detailed monitoring of the operating states of the individual LED clusters. This allows simple fault identification (check for short-circuit, interruption) by sequential sampling (so-called LED scanning) of the individual LED cluster.
In addition, the large series resistor Rv which has been required until now to set the current for the LED cluster is avoided. A 12 V car battery may be mentioned as an example, to which an LED cluster is connected having four LEDs of the Power TOPLED type (U=2.1 V typical) . With conventional current adjustment, this would result in a power loss in the current adjustment resistor Rv of about 250 mW. In contrast, the arrangement according to the invention results in a power loss in the shunt resistor RShunt of only about 5 mW (when PWM is used for current adjustment), that is to say a reduction in the power loss by a factor of 50.
A further advantage is simple current limiting in an LED cluster using a current-limiting semiconductor switch (preferably a transistor). A current-limiting power switch may also be used as the switch, which automatically ensures that the pulsed forward current IF does not exceed a maximum limit value, for example a limit value of 1 A.
The circuit arrangement according to the invention is suitable for various requirements, for example for a 12 V or else 42 V motor vehicle power supply system.
FIG. 5 shows, as a snapshot, an oscilloscope display of the pulsed current profile of the LED drive circuit for a 12 V vehicle power supply system. This shows the peak current iLED through the LEDs (FIG. 5a), which is pulsed and reaches about 229 mA. The pulse width is about 30 xcexcs, and the subsequent dead time 70 xcexcs. This results in a mean current {overscore (i)}LED of 70 mA.
Furthermore, FIG. 5b shows the associated clock frequency at the triangle waveform generator, whose frequency is about 9.5 kHz (corresponding to a pulse width of about 100 xcexcs) . The regulation voltage UReg is shown as a straight line (FIG. 5c), and has a value of 3.2 V.
The large series resistor Rv which has been required until now for current adjustment is thus avoided and is replaced by a small measurement resistor, in the order of magnitude of RShunt=1xcexa9.
Fluctuations in the supply voltage UBatt are now compensated for, and the forward current IF can easily be kept constant. This is because, when the value of the supply voltage changes, the regulation voltage UReg likewise changes, and thus the on-time of the transistor. This pulse-width modulation, in which an increase in the supply voltage results in the transistor on-time being shortened (the same applies in the converse situation) automatically always results in a constant current, which is set on the regulator in the form of a reference voltage URef (see FIG. 4a) Thus, since the forward current IF in the LED cluster is constant, it is also impossible for there to be any more brightness fluctuations when the supply voltages vary.
The circuit arrangement according to the invention allows the temperature to be regulated. According to FIG. 3 (using the example of Power TOPLEDs), the maximum forward current IF of 70 mA in this case must not be kept constant over the entire permissible temperature range (up to an ambient temperature of TA=100xc2x0 C.) . Above an ambient temperature of TA=70xc2x0 C., the forward current IF must be reduced and, at TA=100xc2x0 C., it must finally be switched off. In order to achieve temperature regulation, a temperature sensor (preferably in SMD form) is also fitted in the LED array on the board, to be precise at the point which is expected to be the hottest. If the temperature sensor measures an ambient temperature of at least TA=70xc2x0 C., the forward current IF is reduced in accordance with the specification on the datasheet (FIG. 3). The forward current IF is switched off at an ambient temperature of TA=100xc2x0 C. This temperature regulation measure is necessary in order to protect the light-emitting diodes against thermal destruction from overheating, and in order thus not to shorten their life.
This circuit arrangement allows malfunctions in the LED cluster to be identified easily. If an LED cluster in an LED array (comprising a number of LED clusters) fails, it may be important to signal this failure immediately to a maintenance center. This is particularly important in the case of safety facilities, for example in the case of traffic light systems. Even in the motor vehicle area (passenger vehicles, goods vehicles), it is desirable to be informed about the present status of the LEDs, for example if the tail lights are equipped with LEDs.
The best known fault types are an interruption and a short-circuit. The short-circuit fault type can be virtually precluded with LEDs. If LEDs fail, then, generally, this is due to an interruption in the supply line. An interruption in LED is predominantly due to the influence of heat. This is caused by expansion of the resin (epoxy resin as part of the housing) under the influence of heat, so that the bonding wire which is embedded in it and expands to a different extent (connecting line between the LED chip and the outer pin) breaks.
Another possible destruction mechanism is likewise caused by the influence of heat. Excessive heat softens the resin (that is to say the material of which the housing is composed) which becomes viscous. The chip can become detached, and starts to move. In consequence, the bonding wire can likewise tear.
Thus, in general, mechanical defects (such as tearing of the bonding wire) can be expected as a result of the influence of the severe heating. A circuit for interruption identification in an LED cluster makes it possible to signal the occurrence of a fault to an output (for example a status pin in the case of a semiconductor module). Logic 1 (high) means, for example, that a fault has occurred, while logic 0 (low) indicates the serviceable state.
The drive circuit according to the invention may be produced in the form of a compact LED drive module (IC) which is distinguished by the capability to stabilize the forward current (IF=const.) in LEDs. Further advantages are the external, and thus flexible, forward current adjustment, the low power loss due to switched operation (no need for the large series resistor Rv), the interruption identification in the LED cluster, and the temperature regulation for protection of the LEDs. Another factor is the low amount of current drawn by the LED drive circuit itself (economic standby operation).
In the standby mode, the LED drive module remains connected to a continuous positive (battery voltage in a motor vehicle), although it is switched off, that is to say no current flows through the LEDs. In this state, the drive module itself draws only a small amount of current (intrinsic current consumption tends to 0), in order to avoid loading the battery in the motor vehicle. This is the situation when, for example, the car is parked in a garage or in the open air. Additional current consumption would in this case unnecessarily load the battery. The LED drive module is switched on and off via a logic input (ENABLE input).
In addition, the circuit arrangement can be designed to be resistant to polarity reversal and to provide protection against overvoltages. A polarity reversal protection diode ensures that the LED drive module is not destroyed if it is connected with the wrong polarity to the supply voltage (battery). A combination of a zener diode and a normal diode provides additional protection for the LED drive module against destruction due to overvoltages on the supply voltage pin UBatt.
In one particularly preferred embodiment, a microcontroller-compatible ENABLE input (logic input) is also provided, which allows a microcontroller to be used for drive purposes. The drive module (in particular an integrated circuit IC) for LEDs can thus be integrated in a bus system (for example the CAN bus in a motor vehicle, and the Insta bus for domestic installations).